Tuesday, 1 August 2017

Amber Reveals Prehistoric Parasites Preserved in Blood

Perhaps 20 million years ago, a pair of monkeys in what is today the Dominican Republic sat grooming each other. Somewhere nearby, possibly in the tree where they sat, there was a sticky and probably aromatic collection of plant resin. This wasn’t an unusual jungle scene at the time, but what happened next was.

One of the monkeys discovered a tick feeding and already bloated with its blood meal. The monkey pulled it free. Something punctured the body of the tick – claws perhaps, or teeth - and monkey blood leaked out. Discarded, or dropped by accident (would the monkey not have preferred to eat this tasty morsel?), the tick fell into the resin and stuck there, still oozing blood.

The tick wasn’t the only organism to die in the resin that day. Within the tiny monkey red blood cells, there were piroplasms – parasites that multiply asexually inside blood cells, burst out, and invade new blood cells. Eventually, the process produces male and female forms that can reproduce only after being ingested by a tick, in a blood meal.*

​Fossil Red Blood Cells

The piroplasms ingested by the tick in this story never reproduced. Instead, they became fossilized in amber, along with the blood cells and the tick. All are still identifiable millions of years later. This piece of amber holds the only fossilized red blood cells from a mammal yet found; the parasites within them are an added fascinating treasure.
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Like the tick in this article, this mosquito was discovered in amber in the Dominican Republic.
Image by Didier Desouens CC BY-SA 4.0 (not licensed for upload to Facebook)

Infection with Piroplasms: Babesia, Theileria, Cytauxzoon

Was the monkey sick? Possibly. The destruction of red blood cells caused by the multiplication of the parasites can lead to severe anemia and even death. Babesia bigemina is a notorious killer of cattle in the United States. Theileria parva causes East Coast fever and death in African cattle. Cytauxzoon felis kills domestic cats in the United States. Some infections go unnoticed, however, and animals can develop immunity. It’s possible the monkey wasn’t all that bothered by its infection.

Throughout the ensuing millennia, ticks continued to bite mammals and infect them with piroplasms, and no one even knew it was happening until the early 1880s when Theobald Smith and Frank Kilbourne proved that a tick transmitted Babesia bigemina to cattle. Not only did they solve the mystery of what was causing Texas cattle fever, it was the first proof that creatures such as insects and arachnids (spiders, ticks, mites etc.) could spread diseases with their bites.

​It took about another 75 years before anyone documented a case of piroplasms infecting humans, and then only in humans who lacked a spleen. Caused by Babesia divergens, there were very few such cases, and the disease remained a medical rarity until recently. In 1969, Babesia microti began turning up in the United States in the same places where Lyme disease is emerging. Today, it’s a spreading infection in the United States and Canada, striking thousands of people every year, and killing some.

​Evolution of Babesia

Lots of people still haven’t heard of Babesia, much less piroplasms, and for the rest of us, they still seem quite new. So the discovery of organisms resembling Babesia sp. fossilized in amber that might well be more than 20 million years old is delightful.

Amber is fossilized tree resin.
It has unique properties for preserving
organisms that become embedded in it.
Image by Wibowo Djatmiko CC BY-SA 3.0

For all of that time, they’ve been evolving and adapting to their hosts until we finally became aware of them. We might never have known Babesia has been around that long if those two monkeys hadn’t stopped to pick off a few ectoparasites in the jungle so long ago.

​Photographs published in the report of this finding are tantalizing, though not as clear as a modern-day blood film. What they mostly do is make me want to sit down at a microscope and look at that magnificent piece of amber myself!

​*Researcher George Poinar Jr. can’t be absolutely certain that the animal the tick was feeding on was a monkey, or that the parasites in the blood are piroplasms; however, after considering other evidence – red blood cell and parasite characteristics, the fossil record, knowledge of primates, piroplasms, and other organisms alive today etc. – this is most likely the correct interpretation.